(1) Rapid reaction techniques were used to distinguish the kinetic effects that arise from preincubation with Na ion and K ion on the partial reactions catalysed by (Na ion plus K ion)ATPase. Results from brief exposure to K ion in the absence of Na ion or prolonged exposure to K ion in the presence of Na ion suggest that Na ion and K ion binding sites could be simultaneously occupied with the implication that these species might be cotransported. (2) The age-dependent deficit in the inotropic response of the myocardium to ouabain was found to be at a site distal to the binding of the cardiac glycoside to the (Na ion plus K ion)ATPase receptor. (3) Measurement of the rapid kinetics of Ca ATPase in cardiac and skeletal muscle sarcoplasmic reticulum suggest an explanation for the slower relaxation rate of the cardiac muscle. (4) Studies of the regulation of Ca 2ion uptake by sarcoplasmic reticulum from young and aged animals were continued.